1. Field of the Invention
This invention relates to a process for solvent dewaxing waxy oils. More particularly, this invention relates to a continuous, solvent dewaxing process and apparatus wherein a waxy oil is prediluted with a non-autorefrigerative dewaxing solvent, with the prediluted oil, at a temperature above its cloud point, being then fed to a chilling zone comprising a vertical, staged tower operating continuously at essentially constant pressure. In the chilling zone wax is precipitated from the oil to form a waxy slurry and the so-formed slurry is further cooled down to wax filtration temperature by contact with a liquid autorefrigerant injected into a plurality of said stages, said liquid autorefrigerant evaporating in each of said stages so as to maintain an average slurry cooling rate of from 0.1.degree. to 20.degree. F. per minute and an average temperature drop per stage of from about 2.degree. to 20.degree. F. The dewaxed oil-containing slurry is then fed to wax filters. This process is particularly useful for dewaxing wax-containing lubricating oil fractions and the like.
2. Description of the Prior Art
It is well known in the art to dewax wax-containing hydrocarbon oils, particularly the lube oil fractions of petroleum oil, in order to remove at least a portion of the wax therefrom to obtain a dewaxed oil of reduced cloud and pour points. The most common method of removing the wax or waxy constituents from waxy hydrocarbon oils is via the use of various solvent dewaxing processes. In solvent dewaxing processes the temperature of the wax-containing oil is lowered sufficiently to precipitate the wax therefrom as solid crystals of wax. At the same time, solvents are added to the waxy oil in order to improve the fluidity and reduce the viscosity thereof so that various filtration or centrifugation processes can be used to separate the solid particles of the wax from the dewaxed oil. Strong wax antisolvents (weak oil solvents) such as MEK are often added to decrease wax solubility in the oil/solvent mixture while strong oil solvents (weak wax antisolvents) such as MIBK or toluene are used to modify the solubility characteristics of the solvent so as to allow wax precipitation, while at the same time avoiding oil immiscibility at wax separation temperatures. Solvent dewaxing processes produce what is known as a pour-filter temperature spread. This is the temperature differential between the wax filtering temperature and the pour point of the dewaxed oil. This pour-filter temperature spread is greater when more non-polar hydrocarbon solvents are used than with more polar solvents such as ketones. Thus, an autorefrigerant dewaxing process employing propane can produce a pour-filter spread of 40.degree. F., which means that the wax filtration must be done at -40.degree. F. in order to produce a dewaxed oil having a pour point of 0.degree. F. When ketones or mixtures of ketone and aromatic solvents are used, the pour-filter spread may range from 0.degree. F. to 20.degree. F. depending on the oil and solvent used.
Commercially successful processes employing autorefrigerative cooling, wherein the waxy oil is mixed with a liquid autorefrigerant which is permitted to evaporate thereby cooling the oil by the latent heat of evaporation, are batch or semi-batch operations. This mixture of liquid autorefrigerant and oil are introduced into an expansion chamber wherein the pressure is slowly reduced to achieve controlled evaporation of the autorefrigerant and controlled cooling of the oil, thus avoiding the shock chilling which would result if the autorefrigerant were allowed to flash off. However, batch processes are cumbersome, difficult to operate and energy inefficient.
A number of attempts have been made to develop a continuous autorefrigerant process of dewaxing oils, including combinations of ketone/autorefrigerant processes. Thus, U.S. Pat. No. 3,549,513 discloses an autorefrigerative batch dewaxing process that is described as continuous, but which really operates via the sequential use of a multiple number of batch chillers or expansion chambers. Waxy oil is diluted with an aromatic/ketone solvent mixture and with liquid autorefrigerant and cooling is achieved by controlled evaporation of the autorefrigerant by reducing the pressure in each batch chamber in a manner such that the autorefrigerant evaporates at a controlled rate. U.S. Pat. No. 3,685,688 discloses an autorefrigerant dewaxing process wherein a portion of the wax is precipitated from the oil in a DILCHILL* dewaxing tower wherein the cooling occurs by the injection of cold autorefrigerant into the tower to produce a waxy slurry, followed by autorefrigerative cooling of the slurry in batch chillers. U.S. Pat. No. 2,202,542 suggests a continuous autorefrigerant dewaxing process wherein a waxy oil above its cloud point is premixed with warm, liquid propane. This mixture is introduced into a multi-staged cooling tower and liquid CO.sub.2 is injected into each stage out of direct contact with the oil. This patent emphasizes the point that the liquid CO.sub.2 must be introduced into each stage out of direct contact with the oil in the tower in order to avoid shock chilling. However, this is impractical because the vapor loads on the tower would be far in excess of what could be accommodated in a reasonably sized commercial tower. Also, refrigeration requirements are three times those normally needed and conditions for nucleation and growth of wax crystals are poor. U.S. Pat. No. 3,720,599 discloses a continuous process for dewaxing a waxy petroleum oil stock wherein the oil is premixed with acetone. This mixture is then introduced into a horizontal, elongated chilling vessel containing a plurality of stages operating at different pressures, with the pressure in each stage controlled by a back pressure regulator on each stage. Liquid autorefrigerant is introduced into the stages along the length of the chilling vessel while maintaining a high degree of agitation therein to avoid shock chilling. The autorefrigerant is partially evaporated in each stage, with the amount of evaporation being controlled by the pressure in each stage. Unfortunately, there are problems which currently preclude commercialization of this process, not the least of which is a practical, efficient way of getting the slurry to flow from stage to stage without plugging up the entire apparatus with wax or without multiple transfer pumps which would be expensive and would also tend to destroy the wax crystal structure. Another disadvantage entails the impracticality of providing separately driven agitators of each stage and the mechanical difficulties associated with a common horizontal drive shaft. FNT *Registered service mark of Exxon Research and Engineering Co.
It would be an improvement to the art if one could devise an autorefrigerant solvent dewaxing process that operates in a continuous manner and without the need for mechanical agitators or chambers controlled at separate pressure levels in the chilling zone or tower.